Quaternary structure of intestinal maltase–glucoamylase in pancreatectomized rats

Abstract

Detergent-solubilized intestinal maltase–glucoamylase was isolated 1 week postpancreatectomy (dMpanc) and purified in the presence of detergent and protease inhibitors. Upon sodium dodecyl sulfate – polyacrylamide gel electrophoresis under nondissociating conditions, the major band had a molecular weight of 280 000, slightly smaller than similar bands from detergent (dM) and papain (pM) solubilized maltase from nonpancreatectomized rats. Upon octyl-Sepharose CL-4B chromatography, 57% of the enzyme was eluted by aqueous buffer, unlike pM which was almost completely eluted or dM, 95% of which bound to the column. All fractions of dMpanc from octyl-Sepharose 4B were reduced, by boiling ± β-mercaptoethanol, to monomelic subunits, indicating that processing by pancreatic enzymes at the level of the brush border is not a requirement for the appearance of subunits in the rat. As well, under these dissociating conditions, the 145 000 subunit previously identified with the apolar terminus was present in all fractions of dMpanc, including the aqueous fraction, whereas pM contained only the 130 000 subunit. The presence of dMpanc in the aqueous fraction cannot be explained, therefore, by proteolytic cleavage of an apolar anchor segment from the 145 000 subunit. Pancreatic enzymes may affect the enzyme in a minor fashion, however, since aqueous solubility was enhanced and the apparent molecular weight was reduced by pancreatectomy, suggesting a more compact conformation with shielding of apolar segments.